Preheat oven to 355F.
In a large saucepan or medium sized pot, over medium heat, heat the milk, lemon peel, orange peel, butter, salt and deseeded vanilla bean.
Heat the ingredients until just before they come to a boil. Remove the lemon. orange peel and vanilla bean. Turn the heat down to low, and add in the semolina.
Stir for about 4-5 minutes until thickened. It should be the consistency of creamy oatmeal.
Pour into a bowl and set aside to cool.
In a small bowl, with your hands mix together the seeds from the vanilla bean and granulated sugar.
In a large bowl, with a hand mixer or KitchenAid stand mixer, whisk together the eggs and vanilla sugar until light in color and creamy - this will take a few minutes.
Add in the ricotta and limoncello if using. Whisk again until well incorporated.
Add in the semolina mixture a bit at a time, and continue whisking until well incorporated.
Butter a 9 inch spring form pan. Pour the mixture into the pan.
Bake for approximately 1 hour. By the time you remove the cake from the oven, it will still be a bit jiggly in the centre - this is normal and it will stiffen as it sits.
Let it come to room temperature. Remove from the spring form pan. Top with powdered sugar and enjoy!